1. Field of the Invention
The present invention relates to a TFT (thin film transistor) protection circuit and, more specifically, to a TFT protection circuit for protecting a TFT, which drives pixel electrodes arrayed in matrix in a liquid crystal display or the like, from being damaged due to a surge of electrostatic charge.
2. Description of the Related Art
A flat-faced display device such as a liquid crystal display is used widely in the field of personal computers, wordprocessors, and the like. The liquid crystal display has features of being thin, light, and operable at a low voltage, and its constitution can be modified easily for displaying color images. Attention has recently been attracted to an active matrix liquid crystal display. This liquid crystal display is the most suitable for full-color television and office automation apparatuses since it is free from deterioration of contrast, response or the like due to an increase in pixels and capable of halftone display.
As a basic constitution of a generally-used liquid crystal display, the active matrix liquid crystal display includes a pair of substrates having an array substrate and its opposite substrate, and a liquid crystal cell held between these substrates. The array substrate has a transparent glass substrate, a matrix array of pixel electrodes formed on the glass substrate, a plurality of scanning lines formed along the rows of the pixel electrodes, and a plurality of data lines formed along the columns thereof. The opposite substrate has a color filter and a common electrode which are formed on a transparent glass substrate to face the matrix array of pixel electrodes. In particular, the active matrix liquid crystal display is provided with a plurality of TFTs serving as switching elements for selectively driving the pixel electrodes of the array substrate. Each TFT is located near a crossing point of one of the scanning lines and one of the data lines in order to drive one of the pixel electrodes, and includes a gate electrode connected to the one scanning line, a drain connected to the one data line, and a source connected to the one pixel electrode. The gate electrode is formed on the glass substrate, and the source and drain are formed in an a-Si (amorphous silicon) layer which is formed over the gate electrode with a gate insulation film inserted therebetween.
In the liquid crystal display described above, a selection signal is supplied in sequence to every scanning line to turn on a set of TFTs connected to the scanning line concurrently. When the TFTs are turned on, each TFT supplies a pixel data signal received from a corresponding data line to a corresponding pixel electrode. The orientation of liquid crystal is controlled by a difference in potential between the pixel electrode and the common electrode within a pixel region defined by these electrodes. The liquid crystal display therefore displays images whose light intensity distribution varies with the transmittance distribution. This technique is disclosed by T. P. Brody et al. "IEEE Transactions on Electronic Devices", Vol. ED-20, November, 1973, pp. 995-1001.
Since the display characteristic of the foregoing liquid crystal display greatly depends upon the performance of the pixel electrode driving TFTs, the management of a manufacturing process is important in order to keep an electron mobility .mu.n, a threshold voltage vth, and an off current Ioff, which are principal factors for determining the performance of the TFTs, to desired values. However, these TFTs are weak to a surge of electrostatic charge, like MOS transistors of a silicon IC. If a difference in potential between the gate electrode and one of the source and drain is increased by the electrostatic charge produced in a process of forming an array substrate, the insulation properties of the gate insulation film are degraded, with the result that the TFTs cannot be employed.
Conventionally, a TFT protection circuit, as shown in FIG. 1, is formed on an array substrate before formation of electrode driving TFTs. This protection circuit comprises a conductive outer short-circuit line 10 surrounding an array of the driving TFTs on the array substrate, and a plurality of impedance elements 11 connected between the outer short-circuit line 10 and one of wiring lines such as scanning lines 3 and data lines 4. Each of the impedance elements 11 includes two discharging TFTs 11a and 11b of a-Si (amorphous silicon). The current paths of these discharging TFTs 11a and 11b are connected in parallel to each other, the gate electrode of the TFT 11a is connected to the outer short-circuit line 10, and the gate electrode of the TFT 11b is connected to the wiring line 3 or 4. If a difference in potential between the wiring line 3 or 4 and the outer short-circuit line 10 increases, one of the TFTs 11a and 11b is turned on to transfer the electrostatic charge on the wiring line 3 or 4 to the outer short-circuit line 10. A high voltage is thus prevented from being applied to gate insulation films of the driving TFTs. The TFT protection circuit is used throughout processes of forming the array substrate, of forming a liquid crystal cell, and of assembling a module, and then maintained even after the finished product is obtained. For this reason, the driving TFTs can be protected from its breakage due to a surge of electrostatic charge not only during these processes but also after shipping of the finished products.
However, the above-described TFT protection circuit has the following drawback. If the discharging TFTs 11a and 11b are broken by a surge of electrostatic charge, the insulation property of the gate insulation films deteriorates between the gate electrode and one of the source and drain, resulting that the wiring line 3 or 4 is permanently short-circuited with the outer short-circuit line 10. The gate insulation films normally withstand a voltage of about 100 v, and a voltage applied thereto by electrostatic charge often exceeds the withstand voltage. Generally, a liquid crystal display is considered to be defective simply because the above short circuit occurs on one of 1000 or more wiring lines.
Furthermore, it is desirable that the impedance of the discharging TFTs 11a and 11b be as low as possible to such an extent that it cannot influence the driving of pixel electrodes in order to improve in discharge efficiency of electrostatic charge. Therefore, the impedance is practically set to about ten times as high as the output impedance of driver ICs, which are connected to terminal sections 5 and 6 to supply a selection signal and a pixel data signal. Since, however, the discharging TFTs 11a and 11b are constituted of a-Si (amorphous silicon) whose electron mobility is low, they have to occupy a considerably large area on the array substrate. This restricts the shape of the liquid crystal display or intervals of the wiring lines, with the result that a short circuit may easily occur between adjacent wiring lines during their forming process, thereby lowering the yield of the liquid crystal display.
As described above, the conventional liquid crystal display has the drawback wherein a defect in wiring lines is easy to occur owing to electrostatic charge produced in the manufacturing process of the display since sufficient measures are not taken against the electrostatic charge which causes TFT properties to deteriorate. The drawback is not limited to the above liquid crystal display but applied to various types of display device using TFTs as switching elements.